In magnetic recording, the use of disks having either particulate or thin film magnetic coatings on aluminum substrates, is well known. Each of these types of coatings has problems associated therewith which render them less than totally desirable, particularly for extremely thin coatings which are required for high bit density recording patterns being used now and in the future.
Particulate magnetic coatings on aluminum substrates, especially in extremely thin layers, often exhibit limited durability with respect to contact with the magnetic transducer associated with the disk. Thin film magnetic disks, usually produced by sputtering, evaporation or electroplating of a magnetic coating on an aluminum substrate, generally provide magnetic coatings which are thinner than those available with particulate coatings. However, thin film disks often exhibit significant corrosion problems, primarily because of the reaction between the metal substrate, which has been required to be aluminum because of weight considerations, and the deposited thin film layer. Such disks have required the deposition of one or more protective layers between the substrate and the thin film magnetic layer and over the thin film layer. Understandably, the requirement for these protective layers has added significantly to the cost of the thin film disk. Additionally, both particulate and thin film magnetic disks usually have employed a substrate of substantial thickness relative to the coating thickness, thereby adding appreciably to the weight of an assembly which includes several of such disks.
Since thin films of metals or polymers typically replicate the substrate surface, the necessity to try to achieve surface perfection (i.e. asperity-free, flat, etc., to prevent amplitude modulation and head crashes) in aluminum substrates has been a traditional problem. While improvements in substrate polishing and coatings have been made, future major performance improvements will rely on enhancements of the substrate surface.